Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 487 



firmly attached along the edges. The attachment is so strong that the central 

 part can be pulled away leaving a ring of torn tissue behind it. Each piece will 

 develop tentacles and a mouth and finally a complete minute anemone, ulti- 

 mately a ring of little anemones. 



Anemones can glide on the pedal disk, but at the slow pace of about four 

 inches per hour. When conditions are good they stay in one place for long 

 periods. An anemone contracts its body tightly and quickly; the tentacles dis- 

 appear suddenly, and its mouth appears tied up like a bag. Water is squeezed 

 out through pores in its body wall and the acontia are also forced out through 

 them. It may not expand for a long time and then very slowly while water 

 gradually flows into the enteron through smooth ciliated furrows on one or 

 both sides of the gullet (Figs. 23.1 and 23.16). 



The tentacles are very sensitive to stimulation and move excitedly if meat 

 juices are added to the surrounding water. If a water flea happens to come in 

 contact with the tentacles it is immediately snared in the sticky mucus, then 

 paralyzed by the stinging cells and brought to the mouth by the ciliated ten- 

 tacles (Fig. 23.17). Immediately the whole oral disk is in motion, the mouth 

 opens and, with the further help of tentacles and lips, it takes in the food. In 

 the gullet, the food comes in touch with currents of cilia, always inward when 

 the anemone is feeding though they may be outward at some other times. 

 Anemones are carnivores that will eat any animal flesh, living or dead. They 

 often attach themselves to crab shells and to the shells appropriated by hermit 

 crabs. The crab is hidden and the sea anemone rides to new feeding grounds, 

 foraging as it goes, probably a truly symbiotic relation. 



Astrangia — A Coral Polyp. Astrangia danae form little colonies of a couple 

 of dozen polyps on the rocks, in sheltered places from North Carolina to 

 Massachusetts (Fig. 23.18). They feed upon small crustaceans and young 

 fishes and can be kept alive quite successfully in cold salt-water aquaria. They 

 are like smaller editions of the sea anemone except for the limy coral cup 

 secreted by the ectoderm. This is laid down at the base of the polyp, in thin 

 ridges and as more coral is produced the bottom of the cup is also thickened. 

 Astrangia is closely related to the most important builders of coral reefs. 



Coral Building. In tropical waters, where they abound, coral animals have 

 built the foundations of large areas of land. The Bermuda Islands are at the 

 northern limit of coral building and are comparatively small, yet they include 

 more than 19 square miles of coral. The Great Barrier Reef of Australia, 

 crowded with coral, is 1350 miles long (Fig. 23.20). Such areas have been 

 built by the epidermal cells of millions of minute polyps each one slowly 

 secreting its cup-shaped home. Polyps die and new generations of them secrete 

 new cups upon the old ones. Only the surface of the coral mass is alive. 



Other animals live in the crevices and chasms of the coral ledges — pro- 

 tozoans, sponges, boring mollusks, case-making worms, seaweeds, and bril- 



